In general, a cryogenic assembly is described in WO 03/044424 A2 for cooling superconducting elements, said cryogenic assembly being suitable for a plurality of low temperature applications such as magnetic resonance imaging, superconducting transformers, fault current limiters etc.
The superconducting elements are housed in an interior vessel filled with the cryogenic medium, i.g. liquid helium. The interior vessel is enclosed in a cryostat with an insulating evacuated intermediate space defined by the outer wall of the cryostat housing and the outer wall of said interior vessel.
For electrical connection of the superconducting elements with the warm surrounding of the cryostat a neck tube is provided extending from the interior vessel into the evacuated intermediate space of the cryostat. Further, insulated external terminals are provided in the outer wall of the cryostat for allowing passage of an electric cable for electric connection of the neck tube with the warm surrounding of the cryostat. By this assembly electrical connection of the superconducting elements within the interior vessel with the warm surrounding of the cryostat is possible via the neck tube without the need of opening of the neck tube which is to be avoided in order not to admit air to the interior vessel.
In particular, the present invention relates to an electric current limiter comprising                a cryogenic vessel having a main body and a cover, both defining an inner space separated from a surrounding space,        superconducting elements of the HTSC type immersed in a cryogenic liquid contained in the inner space of the vessel,        an ullage space between the cryogenic liquid an the cover,        and bushings with conductors extending from the inner space of the vessel to the surrounding space, the conductors connecting the superconducting elements with a current path to be protected against a fault current.        
A current limiter of the HTSC-type mentioned before, where HTSC stands for High Temperature Superconductor, is described in a publication entitled “Fault current limiter in medium and high voltage grids” by Dr.-Ing. Martin Kleimaier and Prof. Dr.-Ing. Claus Neumann that was presented at “IEA Workshop on Electricity Transmission and Distribution Technology and R&D” in Paris, France, on 4-5 Nov. 2004. Also in this publication there is described the technical background for understanding the general purpose and the considerable value of current limiters in transmission and distribution networks.
The cryogenic vessel of the superconducting current limiter according to the publication mentioned above is closed to the surrounding atmosphere by a cover that carries all the elements necessary for the operation of the device such as pipes for transferring a cooling liquid into the vessel for maintaining the superconducting state and bushings having conductors for the connection of the superconducting elements inside the vessel to an external circuit which is to be protected against a fault current. In case the current limiter has to undergo inspection or service work at least all electrical connections between the conductors of the bushings and the external circuit have to be dismantled prior to removing the cover of the cryogenic vessel. This is necessary in contrast to pipes or hoses used for carrying the cooling liquid which can be made from flexible material because conductors adapted to a high fault current are exposed to considerable mechanical forces and thus can only be designed as solid parts.